1. Field of the Invention
The present invention relates to a novel compound, a liquid crystal composition containing the compound, an optical anisotropic material obtained by polymerizing the liquid crystal composition, and an optical device.
2. Discussion of Background
An optical device to modulate (e.g. polarize, diffract or phase-adjust) laser light, is required for an optical head device to read out information recorded on an optical disk or to write information on an optical disk.
For example, as shown in FIG. 3, at the time of reading out information, linearly-polarized light emitted from a light source 1 reaches an information recording surface of an optical disk 6 via a beam splitter 2, a collimator lens 3, a retardation plate 4 and an objective lens 5. The polarization direction of the outward linearly-polarized light is changed to circularly-polarized light by the retardation plate 4 after linearly passing through the beam splitter 2. Such circularly-polarized light is reflected on the information recording surface of the optical disk 6 and becomes inversely rotating circularly-polarized light, which returns via the objective lens 5, the retardation plate 4 to the collimator lens, inversely to the outward path. In this homeward path, the light is changed by the optical retardation plate 4 to linearly-polarized light perpendicular to the polarization before incidence. This homeward light is changed in the direction of the linearly-polarized light by 90° from the outward light, whereby when it passes through the beam splitter 2, its traveling direction is bent and then reaches the optical detector 7.
If plane fluctuations, etc. of the optical disk occur at the time of reading out or writing information, the focus point of the beam spot is likely to be displaced from the recording surface. In order to detect and correct such displacement and to let the beam spot follow a concavoconvex pit on the recording surface, a servomechanism not shown will be required. Such a servomechanism for an optical disk is designed to focus a beam spot irradiated from a laser light source on the recording surface and then to detect the position of the track thereby to follow the desired track. Further, it is also necessary to design so that laser light reflected without hitting the pit on the recording surface, will not return to the light source as it is.
The structure of the optical head device in FIG. 3 shows only the basic structure. There may be cases where other known structures are incorporated, such as a diffraction grating to generate three beams for tracking, an aperture-controlling device to meet light sources with plural wavelengths, a focus-controlling device to meet plural layers of information recording surfaces, and an aberration-correcting device.
Accordingly, in an optical head device, an optical device is required to modulate (e.g. polarize, diffract or phase-adjust) laser light from a light source. For example, a retardation plate (wavelength plate) has a function to present different refractive indices to incident light depending upon the angle between the optical axis of the retardation plate and the phase plane of the incident light and further has an effect to present a phase difference between two component lights formed by birefringence. The two lights having a phase lag will be combined when they come out from the retardation plate. This phase lag is determined by the thickness of the retardation plate, and accordingly, it is possible to prepare e.g. a ¼ wavelength plate to present a phase lag of π/2 or a ½ wavelength plate to present a phase lag of π, simply by adjusting the thickness. For example, linearly-polarized light passed through the ¼ wavelength plate will be circularly-polarized light, and a linearly-polarized light passed through the ½ wavelength plate will be linearly-polarized light with a polarization phase shifted by 90°. By utilizing such characteristics, optical devices are suitably combined and applied to a servomechanism, etc. Such an optical device is used not only in an optical head device used to read out a record on an optical disk but also in an imaging device to be used for e.g. a projector or in a communication device to be used for e.g. a wavelength-variable filter.
Further, such an optical device may be prepared also from a liquid crystal material. A liquid crystal molecule having a polymerizable functional group has both a characteristic as a polymerizable monomer and a characteristic as liquid crystal. Accordingly, by carrying out polymerization after aligning liquid crystal molecules having polymerizable functional groups, it is possible to obtain an optical anisotropic material having alignment of liquid crystal molecules fixed. Such an optical anisotropic material has an optical anisotropy such as a refractive index anisotropy derived from a mesogen skeleton, and it is applied to a diffraction device, phase-plate or the like by utilizing such characteristics.
As such an optical anisotropic material, high molecular weight liquid crystal has been reported which is obtained by polymerizing a liquid crystal composition containing a compound of the following formula (3) (provided that Z in the formula is an alkyl group) (Patent Document 1).CH2═CH—COO-Ph-OCO-Cy-Z  (3)
The optical device is usually required to have the following characteristics.
1) It has a proper retardation value (Rd value) depending upon the wavelength used or application.
2) The in-plane optical characteristics (Rd value, transmittance, etc.) are uniform.
3) Scattering or absorption is not substantially observed at the wavelength to be used.
4) The optical characteristics can easily be adjusted with other materials constituting the device.
5) The wavelength dispersion of the refractive index or refractive index anisotropy is small at the wavelength to be used.
It is especially important to have a proper Rd value as mentioned in 1). The Rd value is defined by Rd=Δn (value of refractive index anisotropy)×d (d is the thickness in light transmission direction), and accordingly, it becomes particularly important that the material constituting the optical device has a proper Δn value. For example, in a case where Δn is small, it is necessary to increase the thickness d. However, if the thickness d is increased, alignment of liquid crystal tends to be difficult, and it becomes difficult to obtain desired optical characteristics. On the other hand, in a case where the Δn value is large, it becomes necessary to reduce the thickness d. However, in such a case, it becomes difficult to precisely control the thickness.
Further, in recent years, in order to enlarge the capacity of an optical disk, it has been attempted to shorten the wavelength of laser light to be used for writing and reading out information and to minimize the concavoconvex pit size on an optical disk. At present, a laser light with a wavelength of 780 nm is used for CD, a laser light with a wavelength of 650 nm is used for DVD, and a laser light with a wavelength of 405 nm is used for BD or HDDVD. It is possible to further shorten the wavelength for next generation optical recording media, and use of laser light with a wavelength of from 300 to 450 nm (hereinafter referred to also as blue laser light) is expected to further increase in the future. However, the conventional material such as high molecular weight liquid crystal disclosed in Patent Document 1 had a problem that the durability against blue laser light was inadequate.
For example, when an optical device (such as a retardation plate) made of an organic substance such as liquid crystal is disposed in an optical system and used as an optical head device, an aberration may sometimes occur as the time passes. This trouble is considered to be attributable to a fact that the organic substance is damaged by exposure to blue laser light. Once such an aberration occurs, it is likely that when light (luminous flux) emitted from a light source, passed through a collimator lens or an optical device or the like and further passed through an objective lens, reaches a recording medium surface, the luminous flux will not focus at one point, and the efficiency to write or read out information (the utilization efficiency of light) tends to deteriorate.
Further, in order to reduce the size and improve the efficiency of the device, a material having high refractive index anisotropy is required. Generally, a material having high refractive index anisotropy tends to have a high refractive index. Further, a high refractive index material has a characteristic such that the wavelength distribution of the refractive index is large, and it tends to have a high light absorption for light with a short wavelength (i.e. the molar absorption coefficient of the material tends to be large).
Therefore, a conventional high refractive index material has had a problem that it tends to absorb light with a short wavelength such as blue laser light, and its light stability is low. To solve such a problem, a material having a small molar absorption coefficient is desirable, and a compound having a totally alicyclic structure as a cyclic structure containing no aromatic ring may, for example, be mentioned. However, such a totally alicyclic liquid crystal monomer usually has a small birefringence anisotropy (Δn), and when it is made into a polymer, Δn will be further smaller or isotropic, whereby there has been a problem that a desired liquid crystal property can hardly be obtainable. Specifically, the following compound (4-1) or (4-2) may, for example, be mentioned, but although the monomer exhibits optical anisotropy (birefringence), it becomes an isotropic polymer when polymerized.CH2═CH—COO-Cy-Cy-C3H7  (4-1)CH2═CH—COO-Cy-Cy-C5H11  (4-2)
For an optical device such as an diffraction device or a retardation plate to modulate laser light with a wavelength of from 300 to 450 nm, an optically anisotropic material is desired which is excellent in durability with little deterioration even when exposed to laser light within this wavelength region and which is excellent in liquid crystallinity, and the structure of a liquid crystal monomer becomes very important.
Patent Document 1: JP-A-2004-263037